This invention relates to a dense single phase membrane having both high ionic and electronic conductivity and capable of separating oxygen from an oxygen containing gaseous mixture and further use of the membrane.
Inorganic membranes are beginning to show promise for use in commercial processes for separating oxygen from an oxygen containing gaseous mixture. Envisioned applications range from small scale oxygen pumps for medical use to large scale integrated gasification combined cycle plants. This technology encompasses two different kinds of membrane materials, solid electrolytes that are mixed conductors and ionic conductors. In both cases the transport is by anionic vacancies or interstitial defects in the electrolyte. In the case of pure ionic conductors, electrons have to be transported in an external circuit, while in the case of mixed conductors no external circuit is necessary as electrons are transported in the membrane material. The driving force for transport is in the mixed conductor case supplied by a difference in partial pressure of oxygen between the two sides of the membrane, while in the pure ionic case in addition an external electrical potential can be supplied.
Membranes formed from mixed conducting oxides which are operated at elevated temperatures can be used to selectively separate oxygen from an oxygen containing gaseous mixture when a difference in oxygen partial pressure exists across the membrane. Oxygen transport occurs as molecular oxygen is dissociated into oxygen ions which migrate to the low pressure side of the membrane and recombine to form oxygen molecules. Electrons migrate through the membrane in the opposite direction to conserve charge. The rate at which oxygen permeates through the membrane is mainly controlled by two factors, the diffusion rate within the membrane and the rate of interfacial oxygen exchange. Diffusion controlled oxygen permeability is known to increase proportionally with decreasing membrane thickness at high temperature (Fick""s law). With decreasing thickness the surface exchange control becomes more important.
During recent years the use of dense mixed conducting membranes in various processes has been described. Examples are oxygen production described in European Patent Application no 95100243.5 (EP-A-663230), U.S. Pat. No. 5,240,480, U.S. Pat. No. 5,447,555, U.S. Pat. No. 5,516,359 and U.S. Pat. No. 5,108,465, partial oxidation of hydrocarbons described in U.S. Pat. No. 5,714,091 and European Patent Application no 90134083.8 (EP-A-438902), production of synthesis gas described in U.S. Pat. No. 5,356,728 and enrichment of a sweep gas for fossile energy conversion with economical CO2 abatement described in the none published international patent Application Nos.: PCT/NO97/00170, PCT/NO97/00171 and PCT/NO97/00172 (Norsk Hydro ASA).
For the application of MCM (Mixed Conducting Membrane) technology, the membrane material must fulfill certain requirements in addition to being a good mixed conductor. These fall into the two categories of thermodynamic and mechanical stability. The membrane material must be thermodynamically stable over the appropriate temperature and oxygen partial pressure range. Furthermore, the membrane material must be stable towards the additional components in the gaseous phase, and towards any solid phase in contact with it (e.g. support material). This calls for different materials for different applications.
Previous reports on oxygen permeable membranes have dealt with perovskite related materials based on the general formula ABO3xe2x88x92xcex4 where A and B represent metal ions. xcex4 has a value between 0 and 1 indicating the concentration of oxygen vacancies. In the idealised form of the perovskite structure it is required that the sum of the valences of A ions and B ions equals 6. Materials known as xe2x80x9cperovskitesxe2x80x9d are a class of materials which has an X-ray identifiable crystalline structure based upon the structure of the mineral perovskite, CaTiO3. Perovskite type oxides ABO3xe2x88x92xcex4 containing dopants on the A and B-site are promising materials for oxygen-permeable membranes. In such materials the oxygen ions are transported through the membrane via oxygen vacancies. Usually the large A-site cation is a trivalent rare earth, while the smaller B-cation is a transition metal (e.g. LaCoO3xe2x88x92xcex4). The trivalent rare earth A-site cation is usually partially substituted by divalent alkaline earth (e.g. Sr), to increase the vacancy concentration, xcex4/3, on the oxygen sub lattice. A similar increase in xcex4 can be accomplished by partial substitution of the B-site cation by a divalent cation (e.g. Zn, Mg), or more commonly by another mixed-valent transition metal (e.g. Fe, Ni, Cu). One of the first reported examples of such a material is La0.8Sr0.2Co0.8Fe0.2O3xe2x88x92xcex4 (Teraoka et al., Chem. Lett. (1985) 1743-1746). European patent application no. 95100306.0 (EP-A-663232) and U.S. Pat. No. 5,712,220 describe compositions of this type for oxygen separation.
When A is divalent and B is trivalent xcex4 will be close to 0.5. A number of these compounds adopt the brown millerite structure where the oxygen vacancies are ordered in layers. Compositions of this type are described in U.S. Pat. No. 5,714,091 and International patent application no.: PCT-US96/14841 for use as membranes in the partial oxidation reactors.
When separating oxygen from an oxygen containing gaseous mixture the membrane is a conductor of product fluid ions and electrons. When no direct oxidation process takes place on the product site of the membrane there is a relatively small difference in partial pressure of oxygen across the membrane, and accordingly the driving force is small. For such applications it is beneficial to use a membrane material where the defects are interstitial oxygen excess, with most of the stoichiometry change in the oxygen partial pressure range in question, rather than oxygen vacancies as in the perovskites. This will ensure a maximum of gradient in oxygen concentration in the material at small oxygen partial pressure gradients. The activation energy for transport of oxygen ions will most often be lower in the case of interstitials than in the case of vacancies.
The main object of the invention was to arrive at a membrane capable of separating oxygen from an oxygen containing gaseous mixture.
Another object of the invention was to arrive at a membrane comprising a material thermodynamically stable over the appropriate temperature and oxygen partial pressure range.
Furthermore, an object of the invention was to arrive at a membrane comprising a material possessing structures that can accommodate interstitial oxygen excess.
Furthermore, another object of the invention was to arrive at a membrane comprising a material showing very low chemical expansion.
Still another object of the invention was to arrive at a membrane stable towards the additional components in the gaseous phase.
Still another object of the invention was to arrive at a membrane stable towards any solid phase in contact with the membrane.
The inventors found that a dense single-phase membrane comprising of a mixed metal oxide material with interstitial oxygen excess represented by the formula:
AyAxe2x80x2yxe2x80x2Axe2x80x3yxe2x80x3BxBxe2x80x2xxe2x80x2Bxe2x80x3xxe2x80x3Bxe2x80x2xe2x80x3xxe2x80x2xe2x80x3O4+xcex4
where A, Axe2x80x2 and Axe2x80x3 are chosen from group 1, 2 and 3 and the lanthanides; and B, Bxe2x80x2, Bxe2x80x3 and Bxe2x80x2xe2x80x3 are chosen from the transition metals according to the periodic table of the elements adopted by IUPAC wherein 0xe2x89xa6yxe2x89xa62, 0xe2x89xa6yxe2x80x2xe2x89xa62, 0xe2x89xa6yxe2x80x3xe2x89xa62, 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6xxe2x80x2xe2x89xa61, 0xe2x89xa6xxe2x80x3xe2x89xa61, 0xe2x89xa6xxe2x80x2xe2x80x3xe2x89xa61 and x and y each represents a number such that y+yxe2x80x2+yxe2x80x3=2, x+xxe2x80x2+xxe2x80x3+xxe2x80x2xe2x80x3=1 and xcex4 is a number where 0xe2x89xa6xcex4 less than 1 quantifying the oxygen excess has both high ionic and electronic conductivity and is capable of separating oxygen from an oxygen containing gaseous mixture.
Furthermore, the inventors found that this membrane was suitable for use for production of pure oxygen, for oxygen enrichment of a sweep gas for fossile energy conversion, for synthesis gas production and for production of oxygen for application in any catalytic or non-catalytic processes wherein oxygen is one of the reactants.
The membrane is especially suitable in applications with high CO2 and high H2O partial pressures. Thus the membrane is suitable for O2-enrichment of a sweep gas containing CO2 for fossile energy conversion with economical CO2 abatement.
Preferably the A, Axe2x80x2 and Axe2x80x3 of the enumerated formula are selected from group 2, 3 or the lanthanide metals. The structure of the complex oxide is such that the d-block metals are nominally six-coordinated by oxygen, forming sheets of oxygen octahedra stacked one above the other. Adjacent sheets are displaced relative to one another by xc2xd xc2xd xc2xd. The lanthanide metals are positioned in between these sheets. In the lanthanide layer interstitial positions are available for excess oxygen.
The structure is usually referred to as the xe2x80x9cKNiF4 -structurexe2x80x9d after the compound KNiF4 (C.N.Rao and I. Gopalakrishnan xe2x80x9cNew Directions in the Solid State Chemistryxe2x80x9d Cambridge University Press 1997).
Preferred mixed conducting dense oxides are represented by the formula La2Ni1xe2x88x92xBxO4+xcex4 wherein x is between 0 and 1 and B is selected from nickel, iron, cobalt and copper. The purpose of the substitution for Ni is mainly to optimize the material for the partial pressures of oxygen in question.
In practice, an oxygen containing gas, such as air, is passed in contact with the solid membrane on one side, the first zone. As the oxygen containing gas contacts the solid membrane, oxygen is reduced to oxygen ions which are transported through the solid electrolyte to the surface on the other side facing the second zone with lower partial pressure of oxygen. At the second zone the oxygen ions are either oxidised to oxygen gas (pure oxygen production) or oxidised and consumed with an enrichment of a sweep gas of H2O and/or CO2 (fossile energy conversion with CO2 abatement). The released electrons at the surface facing the second zone are transported back to the surface facing the first zone via the solid membrane. The total conductivities (ionic and electronic) of the membranes lie in the range 60 to 100 S/cm and the membranes are therefore well suited for such processes. The driving force for the process is the difference in oxygen partial pressure across the membrane which establish an oxygen ion concentration gradient through the membrane.
The oxygen flux through membranes are controlled either by surface kinetic limitations (on one or both sides) or bulk diffusion limitations.
The oxygen flux rates of e.g. the La2Ni1xe2x88x92xBxO4+xcex4 membranes (0.3-4 mm thickness) display a considerable degree of surface exchange control, increasing with decreasing temperature. FIG. 1 shows typical flux rates for pure La2NiO4+xcex4. At the highest temperatures the slopes appear to be similar, but as the temperature is decreased, the slopes experience quite different behaviours which might be an indication that the surface exchange becomes increasingly more important in the oxygen transport process as the temperature decreases. Apparent total activation energies for oxygen flux in the temperature range 900-1000xc2x0 C. were 55-80 kJ/mol which is about 75-150 kJ/mol lower than for known perovskite related materials based on the general formula ABO3 (Carter et al. Solid State Ionics 53-56 (1992)p.597-605). Activation energies for bulk transport and surface exchange were estimated as (40xc2x115) and (100xc2x110) kJ/mol, respectively. The flux rates are weakly dependent on substitution as demonstrated in FIG. 2. The highest rates were found when B was selected from iron and x=0.1, corresponding to 1.0 mlxc2x7cmxe2x88x922minxe2x88x921 at 975xc2x0 C. for a 0.5 mm thick membrane in an oxygen partial pressure gradient of xcex94log(PO2/bar)=2.3 (PO2 =0.5 bar at the feed side). The surface exchange control can be reduced with a catalytic layer on one or both sides, or the surface area can be made larger with a porous layer of the same material. In the latter case the porous layer can also act as a mechanical support to the dense thin membrane (e.g. as described in U.S. Pat. No. 5,240,480).
Another attractive feature of these membrane materials is the low so-called xe2x80x9cchemical expansionxe2x80x9d. When the stoichiometry of a material varies with the oxygen pressure, a volume change with change in oxygen partial pressure is observed.
This effect, referred to as xe2x80x9cchemical expansionxe2x80x9d, causes strain when the material is subjected to an oxygen partial pressure gradient, and thus limits how thin a membrane can be without mechanically cracking. The membrane materials of the present invention show very low chemical expansions, thus minimising this problem.
For the application of separating or recovering oxygen from a gas mixture containing oxygen at elevated temperatures, the membrane material must be thermodynamically stable over the appropriate temperature and oxygen partial pressure range. Furthermore, the membrane material must be stable towards the additional components in the gaseous phase. The membrane materials of the present invention possessing the xe2x80x9cK2NiF4 -structurexe2x80x9d, are suitable as membranes in applications with high CO2 pressures. At 950xc2x0 C. and oxygen partial pressures below 1 bar, the materials are estimated to be stable at CO2 pressures up to 2-6 bar.
FIG. 3 shows a tentative stability diagram at constant temperature (950xc2x0 C.) for La2NiO4+xcex4 in the presence of CO2 and O2. The stability of the La2Ni1xe2x88x92xBxO4+xcex4 materials towards reaction with CO2 increases with increasing temperature. Hence, at e.g. 1000xc2x0 C. (and 1100xc2x0 C.), these materials can be used at CO2 pressures up to approximately 10 and 30 bar respectively. The materials will be stable towards superheated steam with steam pressures in excess of 100 bar above 1000xc2x0 C. This relatively high stability towards H2O and CO2 renders these materials suitable as membranes for O2-enrichment of steam or CO2 containing sweep gases.
The membrane material must be stable towards any solid phase in contact with it such as supporting and sealing materials, at the operating temperature. The thermal expansion coefficient (TEC) of the membrane of the present invention is approximately 14*10xe2x88x926Kxe2x88x921 and match well with different suitable sealing materials according to the none published international patent application no.: PCT/NO97/00169 (Norsk Hydro ASA).
The membrane of the present invention can be used to separate oxygen from an oxygen containing gas or gas mixture. When an oxygen-containing gas with a moderately high oxygen partial pressure is passed along one side of the membrane, oxygen will adsorb and dissociate on the membrane surface, become ionised and diffuse through the solid membrane as interstitial oxygen excess, and desorb as oxygen gas at the low oxygen partial pressure side of the membrane.
The necessary circuit of electrons for this ionization/deionization process is maintained internally in the oxide via its electronic conductivity. Typically mixed conducting oxides demonstrate an oxygen ionic conductivity ranging from 0.01 S/cm to 10 S/cm and an electronic conductivity ranging from about 1 S/cm to 1000 S/cm under operating conditions. A membrane of the present invention, represented by unsubstituted La2Ni1xe2x88x92xBxO4+xcex4, has a typical total (electronic and ionic) conductivity at operating conditions ranging from about 60 S/cm to 100 S/cm in the temperature range 600-1000xc2x0 C.